Chemical/mechanical planarization (CMP) is an essential process in the manufacture of semiconductor chips today. During CMP, the combination of chemical etching and mechanical abrasion produces the required flat, precise surface for subsequent depositions. In the CMP process, the semiconductor wafer is retained in a circular carrier head and pressed against a polyurethane polishing pad covered with a chemical slurry. The pressure exerted on the wafer, the rates of rotation of the platen and the carrier head, the chemical composition of the slurry, the temperature of the environment, and the condition of the polishing pad are all closely controlled. The object is to have a repeatable, consistent process each time so that each wafer is as close as possible to an exact copy of every other wafer.
Therefore, every effort is made in semiconductor manufacture to tightly control all factors of the process in order to insure uniformity of the product. However, one problem has arisen that has not been addressed to date. That is, the semiconductor wafer is retained within the carrier head by a carrier ring that prevents the wafer from exiting the carrier head under the effects of the rotary motion of polishing. The rotation of the carrier head or the rotation of the polishing platen and pad can cause the wafer to exit the carrier head if it is not restrained by the carrier ring. Therefore, these forces may combine to cause the wafer to rotate within the carrier head in a random manner, thereby jeopardizing process repeatability. As each wafer rotates within the individual carrier head at a different rate or amount, the effect is to introduce variability in the CMP process and in the final product. This, of course, increases device failure rates and costs.
Accordingly, what is needed in the art is a device and method for preventing unwanted rotation of the semiconductor wafer within the carrier head during chemical/mechanical planarization.